Dispersion stabilizer for suspension polymerization of vinyl-based compound

ABSTRACT

The present invention aims at providing a dispersion stabilizer for suspension polymerization of a vinyl-based compound, which gives vinyl chloride-based polymer particles having a sharp particle size distribution and a high bulk density owing to excellent dispersion stability during polymerization, and which can produce vinyl chloride-based polymer particles with few wet foams, hardly accompanied with formation of polymer scales and foamy polymers attributable to dry foams, with less coloring, and having excellent heat resistance. The dispersion stabilizer of the invention comprises a PVA-based resin having a 1,2-diol component at a side chain and a degree of saponification of 65 to 87% by mol.

TECHNICAL FIELD

The present invention relates to a dispersion stabilizer for suspensionpolymerization of a vinyl-based compound, in particular, a dispersionstabilizer for suspension polymerization of vinyl chloride. Moreparticularly, the invention relates to a dispersion stabilizer forsuspension polymerization of a vinyl-based compound which gives vinylchloride-based polymer particles having a sharp particle sizedistribution and a high bulk density owing to excellent dispersionstability during polymerization. An aqueous solution thereof is less aptto foam and, hence, the stabilizer is effective in inhibiting theformation of a wet foam during polymerization and diminishing theformation of a dry foam. The stabilizer therefore inhibits the formationof polymer scale, which is causative of fish-eyes, and the formation ofa foamy polymer. Furthermore, with the stabilizer, vinyl chloride-basedpolymer particles with less coloring and excellent in heat resistancecan be produced.

BACKGROUND ART

A general process for industrially producing a vinyl chloride-basedresin is batch type suspension polymerization in which a vinylchloride-based monomer is dispersed in an aqueous medium in the presenceof a dispersion stabilizer and an oil-soluble polymerization initiatoris used to perform polymerization. Factors which govern the quality ofvinyl chloride-based resins generally include a rate of polymerization,a ratio of water/monomer, a polymerization temperature, an amount ofpolymerization initiator, a type of polymerization vessel, a stirringrate, and a kind and amount of the dispersion stabilizer. It is saidthat the dispersion stabilizers are most influential among thesefactors.

Performances required of dispersion stabilizers for suspensionpolymerization of a vinyl chloride-based resin include: (a) to attainsufficient protective colloidal ability and sufficient dispersingability even when used in a small amount and to thereby serve to givevinyl chloride-based polymer particles having a sharp particle sizedistribution; (b) to serve to give vinyl chloride-based polymerparticles which are porous or are inhibited from forming a skin layer soas to increase a rate of plasticizer absorption and thereby facilitatemolding; (c) to serve to give vinyl chloride-based polymer particlesrespectively having porosities which are converged to an almost givenrange so as to remove the vinyl chloride monomer remaining in the porousparticles or prevent from forming fish-eyes or the like in a moldedarticle; and (d) to serve to give vinyl chloride-based polymer particleshaving an increased bulk density so as to improve processing efficiency.

Generally used as the dispersion stabilizers are polyvinyl alcohol-basedresins (hereinafter polyvinyl alcohol is abbreviated to PVA), cellulosederivatives, gelatin, and the like. These are used alone or incombination. Of these, PVA-based resins are most extensively used.However, the PVA-based resins are not considered to fully satisfy therequirements described above, and various attempts are being made inorder to improve the performances thereof.

Recently, a reduction in the time period required for polymerization isdesired for improving productivity. It has been proposed to use apolymerization vessel equipped with a reflux condenser in order toincrease the rate of removing the heat of polymerization reaction.Furthermore, a method in which an aqueous medium which has been heatedbeforehand is added in order to shorten heating time (hot-charge method)has been proposed.

However, in the case of using a polymerization vessel equipped with areflux condenser, there has been a problem that the formation of a wetfoam and dry foam becomes sever because the pressure around the refluxcondenser decreases due to the condensation of the vapor of the vinylchloride-based monomer. The term wet foam means a foam attributed mainlyto the surface-activating ability of the PVA-based resin and mainlycontaining water. The wet foam reduces the effective capacity of thepolymerization vessel and can hence be a factor which reducesproductivity. On the other hand, the term dry foam means a foam whichgenerates mainly in the middle to late period of polymerization andmainly containing vinyl chloride-based resin particles and the vinylchloride-based monomer. The dry foam may become a foamy polymer ordeposit as a polymer scale on the inner wall of the polymerizationvessel and within the reflux condenser. The polymer scale has posed aproblem, for example, that it inhibits heat removal duringpolymerization or comes into the product to cause fish-eyes.

The following have been proposed as measures against such problems: adispersion stabilizer for suspension polymerization of a vinyl-basedcompound comprising a PVA-based polymer which gives an ultravioletabsorption spectrum having an absorbance at 280 nm (a) of higher than0.1 and an absorbance at 320 nm (b) of 0.3 or higher, a value of (a)/(b)of less than 0.3 when in a 0.1% by weight aqueous solution, and whichhas a block character of 0.4 or higher (see, for example, patentdocument 1); a dispersion stabilizer for suspension polymerization of avinyl-based compound comprising a PVA-based polymer obtained bysubjecting a PVA-based polymer having a degree of saponification of 60%by mol or higher and a block character of 0.3 to 0.6 to a heat treatmentat 90 to 180° C. for 0.5 to 20 hours in an atmosphere having an oxygenconcentration of 8,000 ppm or lower (see, for example, patent document2); and a dispersion stabilizer for suspension polymerization of avinyl-based compound comprising a PVA-based polymer which gives anultraviolet absorption spectrum having an absorbance at 280 nm (c) ofhigher than 0.1 and an absorbance at 320 nm (d) of 0.07 or higher, avalue of (c)/(d) of 0.7 or more when in a 0.1% by weight aqueoussolution, which has a YI of 40 or lower when in a 1% by weight aqueoussolution, which has a transmittance, as measured at 30° C. and awavelength of 500 nm, of 80% or higher when in a 0.1% by weight aqueoussolution, and which has a block character of 0.35 or higher and an Mw/Mnof 2.1 to 4.9 (see, for example, patent document 3).

Patent Document 1: JP-A-8-283313 Patent Document 2: JP-A-2004-189888Patent Document 3: JP-A-2004-189889 DISCLOSURE OF THE INVENTION Problemsthat the Invention is to Solve

However, the dispersion stabilizer for suspension polymerizationdescribed in patent document 1 is insufficient in dispersionstabilization although foam forming during polymerization is inhibited.There are cases where use of this stabilizer results in formation ofcoarse particles of the vinyl chloride-based resin. The dispersionstabilizers for suspension polymerization described in patent documents2 and 3 are ones obtained by incorporating double bonds into a PVA-basedresin by a heat treatment or another technique. Because of this, thevinyl chloride-based resin obtained with these stabilizers hasinsufficient heat resistance and hence has problems concerningcoloration and zinc burning which occurs when a barium-zinc-basedstabilizer is used. It was found that there still is room forimprovement.

Namely, there is a desire for a dispersion stabilizer for suspensionpolymerization of a vinyl-based compound satisfying the followingrequirements: the stabilizer attains excellent dispersion stabilizationduring polymerization to thereby diminish the formation of coarseparticles; an aqueous solution thereof is less apt to foam and, hence,it is effective in diminishing the formation of a wet foam and also indiminishing the formation of a dry foam; the stabilizer thus inhibitsthe deposition of a polymer scale, which is causative of fish-eyes, onthe inner wall of the polymerization vessel and further inhibits theformation of a foamy polymer; and vinyl chloride-based polymer particleshaving excellent heat resistance, with less coloring and zinc burning,and having a high bulk density can be produced with the stabilizer.

Means for Solving the Problems

The present inventor diligently made investigations under thesecircumstances. As a result, it has been found that an object isaccomplished with a dispersion stabilizer for suspension polymerizationof a vinyl-based compound which comprises a PVA-based resin comprising a1,2-diol component at a side chain and having a degree of saponificationof 65 to 87% by mol.

The greatest feature of the invention resides in that a PVA-based resincomprises a 1,2-diol component at a side chain, in particular, a primaryhydroxyl group, has been applied to a dispersion stabilizer forsuspension polymerization. This has brought about the effectscharacteristic of the invention.

Incidentally, the PVA-based resin containing 1,2-glycol bonds in anamount of 1.9% by mol or larger described in JP-A-2001-233905 isobtained only by conducting polymerization in an ordinary PVA productionstep at a high temperature and high pressure. These 1,2-glycol bondsindicate a bonding mode of a main chain. The PVA-based resin describedtherein differs from the PVA-based resin according to the invention inthat all the hydroxyl groups thereof are secondary hydroxyl groups.

The PVA-based resin containing a 1,2-diol component at a side chainpreferably is a PVA-based resin which has a 1,2-diol structural unitrepresented by the general formula (1).

[wherein, R¹, R², and R³ each independently represents a hydrogen atomor an organic group; X represents a single bond or a bonding chain; andR⁴, R⁵, and R⁶ each independently represents a hydrogen atom or anorganic group.]

ADVANTAGES OF THE INVENTION

The dispersion stabilizer for suspension polymerization of a vinyl-basedcompound of the invention attains excellent dispersion stability duringpolymerization and, hence, vinyl-based polymer particles having a sharpparticle size distribution and a high bulk density are obtained. Thestabilizer is reduced in wet-form formation and is effective indiminishing polymer scale deposition on the inner wall of thepolymerization vessel and foamy polymer formation, which areattributable to a dry foam. Furthermore, vinyl-based polymer particleswith less coloring and zinc burning, which is occasionally seen inflexible-composition formulations, and having excellent heat resistanceare obtained with the stabilizer. Therefore, the dispersion stabilizerof the invention is exceedingly useful industrially.

Furthermore, the PVA-based resin according to the invention does notnecessitate any heat treatment in the production thereof, in contrast tothe PVA-based resins for use in the related-art dispersion stabilizersfor suspension polymerization, which each necessitate a heat treatmentstep in the production thereof. The dispersion stabilizer of theinvention hence has a large merit also from the standpoint of productioncost.

BEST MODE FOR CARRYING OUT THE INVENTION

The following explanations on constituent elements are for an embodiment(typical embodiment) of the invention and should not be construed aslimiting the invention.

The invention is explained below in detail.

The PVA-based resin to be used in the invention is a PVA-based resinhaving 1,2-diol structural units represented by the following generalformula (1). In the general formula (1), R¹, R², and R³ eachindependently represents a hydrogen atom or an organic group; Xrepresents a single bond or a bonding chain; and R⁴, R⁵, and R⁶ eachindependently represents a hydrogen atom or an organic group.

In this PVA-based resin, a content of the 1,2-diol structural unitsrepresented by the general formula (1) is preferably about 0.3 to 20% bymol. Like ordinary PVA-based resins, the remaining part of thisPVA-based resin comprises vinyl alcohol structural units, which arecontained in an amount corresponding to the degree of saponification,and vinyl acetate structural units as the remainder.

It is desirable that all of R¹ to R³ and R⁴ to R⁶ in the 1,2-diolstructural units represented by the general formula (1) should be ahydrogen atom. However, the hydrogen atom may be replaced with anorganic group in such a degree as not to considerably impair resinproperties. The organic group is not particularly limited. However,alkyl groups having 1 to 4 carbon atoms are preferred, such as a methylgroup, an ethyl group, an n-propyl group, an isopropyl group, an n-butylgroup, an isobutyl group, and a tert-butyl group. It may have asubstituent such as, e.g., a halogen group, a hydroxyl group, an estergroup, a carboxylic acid group, or a sulfonic acid group according toneed.

X in the 1,2-diol structural units represented by the general formula(1) typically is a single bond. However, X may be a bonding chain solong as this does not lessen the effects of the invention. This bondingchain is not particularly limited. Examples thereof include hydrocarbonssuch as alkylene, alkenylene, alkynylene, phenylene, and naphthylene(these hydrocarbons may be substituted with halogen such as fluorine,chlorine, or bromine). Examples thereof further include —O—,—(CH₂O)_(m)—, —(OCH₂)_(m)—, —(CH₂O)_(m)CH₂—, —CO—, —COCO—,—CO(CH₂)_(m)CO—, —CO(C₆H₄)CO—, —S—, —CS—, —SO—, —SO₂—, —NR—, —CONR—,—NRCO—, —CSNR—, —NRCS—, —NRNR—, —HPO₄—, —Si(OR)₂—, —OSi(OR)₂—,—OSi(OR)₂O—, —Ti(OR)₂—, —OTi(OR)₂—, —OTi(OR)₂O—, —Al(OR)—, —OAl(OR)—,and —OAl(OR)O— (wherein R each independently represents any desiredsubstituent, and preferably are a hydrogen atom or an alkyl group; and mis a natural number). Preferred of these from the standpoint ofstability during production or during use is an alkylene group having 6or less carbon atoms or —CH₂OCH₂—.

Processes for producing the PVA-based resin to be used in the inventionare not particularly limited. However, it is preferred to use (i) aprocess in which a copolymer of a vinyl ester-based monomer and acompound represented by the following general formula (2) is saponified.

[wherein, R¹, R², and R³ each independently represents hydrogen or anorganic group; X represents a single bond or a bonding chain; R⁴, R⁵,and R⁶ each independently represents a hydrogen atom or an organicgroup; and R⁷ and R⁸ each independently represents a hydrogen atom orR⁹—CO— (wherein R⁹ represents an alkyl group).]

Besides the process (i), use may be made of:

(ii) a process in which a copolymer of a vinyl ester-based monomer and acompound represented by the following general formula (3) is saponifiedand decarboxylated

[wherein R¹, R², and R³ each independently represents a hydrogen atom oran organic group; X represents a single bond or a bonding chain; and R⁴,R⁵, and R⁶ each independently represents a hydrogen atom or an organicgroup]; or(iii) a process in which a copolymer of a vinyl ester-based monomer anda compound represented by the following general formula (4) is subjectedto saponification and solvolysis of ketal structure therefrom

[wherein R¹, R², and R³ each independently represents a hydrogen atom oran organic group; X represents a single bond or a bonding chain; R⁴, R⁵,and R⁶ each independently represents a hydrogen atom or an organicgroup; and R¹⁰ and R¹¹ each independently represents a hydrogen atom oran organic group].

Examples of the vinyl ester-based monomer to be used in the inventioninclude vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate,vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl caprate, vinyllaurate, vinyl stearate, vinyl benzoate, and vinyl versatate. Of these,vinyl acetate is preferred from the standpoint of profitability.

Those processes (i), (ii), and (iii) are explained below.

[Process (i)]

Process (i) is a method in which a vinyl ester-based monomer iscopolymerized with a compound represented by the general formula (2) andthe resultant copolymer is saponified to produce a PVA-based resinhaving 1,2-diol structural units represented by the general formula (1).

Examples of R¹ to R³, R⁴ to R⁶, and X in the compound represented by thegeneral formula (2) may be the same as those in the general formula (1).R⁷ and R⁸ each independently are a hydrogen atom or R⁹—CO— (wherein R⁹is an alkyl group, preferably a methyl group, a propyl group, a butylgroup, a hexyl group, or an octyl group; this alkyl group may have asubstituent such as, e.g., a halogen group, a hydroxyl group, an estergroup, a carboxylic acid group, or a sulfonic acid group so long as thisdoes not exert an adverse influence on reactivity in thecopolymerization or on the subsequent step).

Examples of the compound represented by formula (2) include compounds inwhich X is a single bond, such as 3,4-dihydroxy-1-butene,3,4-diacyloxy-1-butene, 3-acyloxy-4-hydroxy-1-butene,4-acyloxy-3-hydroxy-1-butene, and 3,4-diacyloxy-2-methyl-1-butene;compounds in which X is an alkylene group, such as4,5-dihydroxy-1-pentene, 4,5-diacyloxy-1-pentene,4,5-dihydroxy-3-methyl-1-pentene, 4,5-diacyloxy-3-methyl-1-pentene,5,6-dihydroxy-1-hexene, and 5,6-diacyloxy-1-hexene; and compounds inwhich X is —CH₂OCH₂— or —OCH₂—, such as glycerin monoallyl ether,2,3-diacetoxy-1-allyloxypropane, 2-acetoxy-1-allyloxy-3-hydroxypropane,3-acetoxy-1-allyloxy-2-hydroxypropane, glycerin monovinyl ether, andglycerin monoisopropenyl ether.

Preferred of those are the compounds in which R¹, R², R³, R⁴, R⁵, and R⁶each are hydrogen, X is a single bond, R⁷ and R⁸ each are R⁹—CO—, and R⁹is an alkyl group, i.e., 3,4-diacyloxy-1-butenes. This is because thesecompounds are superior in reactivity in the copolymerization andindustrial handleability. Especially preferred of these is the compoundin which R⁹ is a methyl group, i.e., 3,4-diacetoxy-1-butene. In the casewhere vinyl acetate is copolymerized with 3,4-diacetoxy-1-butene, thereactivity ratio between these monomers is such that r(vinylacetate)=0.710 and r(3,4-diacetoxy-1-butene)=0.701. This indicates that3,4-diacetoxy-1-butene has excellent reactivity in copolymerization withvinyl acetate as compared with the case of vinylethylene carbonate whichwill be described later, in which r(vinyl acetate)=0.85 andr(vinylethylene carbonate)=5.4.

With respect to 3,4-diacetoxy-1-butene, a product of Eastman ChemicalCompany for industrial production and a product from Acros on a reagentlevel are available on the market. Also usable is the3,4-diacetoxy-1-butene obtained as a by-product in a step for1,4-butanediol production. Furthermore, 3,4-diacetoxy-1-butene may beobtained by a known technique such as, e.g., the method described in,e.g., JP-A-10-212264 in which 1,4-diacetoxy-2-butene is converted to3,4-diacetoxy-1-butene or the method described in WO 00/24702 in which3,4-diacetoxy-1-butene is obtained from 1,3-butadiene via a monoepoxide.

Techniques for this copolymerization of a vinyl ester-based monomer anda compound represented by the general formula (2) are not particularlylimited, and a known technique may be employed, such as, e.g., bulkpolymerization, solution polymerization, suspension polymerization,dispersion polymerization, or emulsion polymerization. In general,however, solution polymerization is conducted.

Methods for adding the monomer ingredients at the copolymerization arenot particularly limited, and any method may be employed, such as, e.g.,adding all at once, adding divisionally, or adding continuously.However, dropping polymerization is preferred because a polyvinylester-based polymer having 1,2-diol structural units derived from thecompound represented by the general formula (2) and evenly distributedin the molecular chain of the polymer can be obtained by the technique.Especially preferred is a method of polymerization based on a HANNAmethod employing the ratio concerning reactivity with vinyl acetatedescribed above. Examples of solvents usable in the copolymerizationgenerally include lower alcohols such as methanol, ethanol, isopropylalcohol, n-propanol, and butanol and ketones such as acetone and methylethyl ketone. Methanol is industrially preferred to be used.

The amount of the solvent to be used may be suitably selected accordingto the target degree of polymerization of the copolymer while takingaccount of a chain transfer constant of the solvent. For example, in thecase where the solvent is methanol, the amount of the solvent to be usedmay be selected so that the S (solvent)/M (monomer) ratio is in therange of about 0.01 to 10 (by weight), preferably about 0.05 to 3 (byweight).

A polymerization catalyst is used for the copolymerization. Examples ofthe polymerization catalyst include known radical polymerizationcatalysts such as azobisisobutyronitrile, acetyl peroxide, benzoylperoxide, and lauryl peroxide and low-temperature-active radicalpolymerization catalysts such as azobisdimethylvaleronitrile andazobismethoxydimethylvaleronitrile. The amount of the polymerizationcatalyst to be used cannot be categorically specified because it variesdepending on the kinds of the comonomers and the kind of the catalyst.However, the amount thereof may be arbitrarily selected according to arate of polymerization. For example, in the case of usingazoisobutyronitrile or acetyl peroxide, the amount thereof is preferably0.01 to 0.7% by mol, especially preferably 0.02 to 0.5% by mol, based onthe vinyl ester-based monomer.

The temperature at which the copolymerization reaction is to beconducted may be in the range of about from 30° C. to the boiling pointaccording to the solvent and pressure to be used. More specifically, thereaction may be conducted at a temperature in the range of 35 to 150°C., preferably 40 to 75° C.

The copolymer obtained is subsequently saponified. In thissaponification, the copolymer obtained above is dissolved in a solvent,e.g., an alcohol, and an alkali catalyst or acid catalyst is used tosaponify the copolymer. Typical examples of the solvent includemethanol, ethanol, propanol, and tert-butanol. However, it is especiallypreferred to use methanol. The concentration of the copolymer in thealcohol may be suitably selected according to the viscosity of thesystem. In general, however, a concentration is selected from the rangeof 10 to 60% by weight. Examples of the catalyst to be used for thesaponification include alkali catalysts such as alkali metal hydroxidesor alcoholates, e.g., sodium hydroxide, potassium hydroxide, sodiummethylate, sodium ethylate, potassium methylate, and lithium methylate,and acid catalysts such as sulfuric acid, hydrochloric acid, nitricacid, methanesulfonic acid, zeolites, and cation-exchange resins.

The amount of the saponification catalyst to be used may be suitablyselected according to the method of saponification, target degree ofsaponification, etc. However, in the case of using an alkali catalyst,an appropriate range of the amount thereof is generally 0.1 to 30 mmol,preferably 2 to 17 mmol based on 1 mol of the sum of the vinylester-based monomer and the 1,2-diol structural units derived from thecompound represented by the formula (4).

The temperature at which the saponification reaction is to be conductedis not particularly limited. However, the reaction temperature ispreferably 10 to 60° C., more preferably 20 to 50° C.

[Process (ii)]

Process (ii) is a method in which a vinyl ester-based monomer iscopolymerized with a compound represented by the general formula (3) andthe resultant copolymer is saponified and decarboxylated to produce aPVA-based resin having 1,2-diol structural units represented by thegeneral formula (1).

Examples of R¹ to R³, R⁴ to R⁶, and X in the compound represented by thegeneral formula (3) to be used in the invention may be the same as thosein the general formula (1). Preferred of such compounds is the compoundin which R¹, R², R³R⁴, R⁵, and R⁶ are hydrogen and X is a single bond,i.e., vinylethylene carbonate. This is because vinylethylene carbonateis easily available and has satisfactory suitability for thecopolymerization.

The copolymerization of a vinyl ester-based monomer and a compoundrepresented by the general formula (3) and the saponification may beconducted in the same manners as in process (i) described above.

With respect to the decarboxylation, it undergoes decarboxylationsimultaneously with the saponification without necessitating any specialtreatment. As a result, the ethylene carbonate ring opens and is thusconverted to a 1,2-diol structure.

It is possible to conduct decarboxylation at a given pressure (ordinarypressure to 1×10⁷ Pa) and a high temperature (50 to 200° C.) withoutsaponifying the vinyl ester moieties. In this case, the saponificationmay be conducted after this decarboxylation.

[Process (iii)]

Process (iii) is a method in which a vinyl ester-based monomer iscopolymerized with a compound represented by the general formula (4) andthe resultant is subjected to saponification and solvolysis of ketalstructure therefrom to produce a PVA-based resin having 1,2-diolstructural units represented by the general formula (1).

Examples of R¹ to R³, R⁴ to R⁶, and X in the compound represented by thegeneral formula (4) to be used in the invention may be the same as thosein the general formula (1). R¹⁰ and R¹¹ each independently are hydrogenor an alkyl group. This alkyl group is not particularly limited.However, the alkyl group preferably is an alkyl group having 1 to 4carbon atoms such as, e.g., a methyl group, an ethyl group, an n-propylgroup, an isopropyl group, an n-butyl group, an isobutyl group, or atert-butyl group. This alkyl group may have a substituent such as, e.g.,a halogen group, a hydroxyl group, an ester group, a carboxylic acidgroup, or a sulfonic acid group so long as this does not inhibitreactivity in the copolymerization, etc. Preferred of such compounds arethe compounds in which R¹, R², R³, R⁴, R⁵, and R⁶ each are hydrogen, Xis a single bond, and R¹⁰ and R¹¹ each are an alkyl group, i.e.2,2-dialkyl-4-vinyl-1,3-dioxolane. This is because these compounds areeasily available and have satisfactory suitability for thecopolymerization. Especially preferred of these is the compound in whichR¹⁰ and R¹¹ each are a methyl group, i.e.,2,2-dimethyl-4-vinyl-1,3-dioxolane.

The copolymerization of a vinyl ester-based monomer and a compoundrepresented by the general formula (4) and the saponification of theresultant may be conducted in the same manners as in process (i)described above.

The solvolysis of ketal structure may be conducted in the followingmanner. In the case where the saponification reaction is conducted usingan alkali catalyst, after the saponification it is further subjected tosolvolysis of ketal structure with an acid catalyst in an aqueoussolvent (e.g., water, water/acetone, or a mixed solvent containing alower alcohol such as water/methanol) to convert into a 1,2-diolstructure. Examples of the acid catalyst in this case include aceticacid, hydrochloric acid, sulfuric acid, nitric acid, methanesulfonicacid, zeolites, and cation-exchange resins.

In the case where the saponification reaction is conducted using an acidcatalyst, it undergoes solvolysis of ketal structure simultaneously withthe saponification without necessitating any special treatment. As aresult, it is converted to a 1,2-diol structure.

The PVA-based resin to be used in the invention may be one in which anyof various unsaturated monomers is copolymerized so long as this doesnot defeat an object of the invention. The amount of such an unsaturatedmonomer to be introduced cannot be categorically specified. However, toolarge amounts thereof are undesirable because there are cases where theresultant has impaired water solubility or reduced gas barrierproperties.

Examples of the unsaturated monomers include olefins such as ethylene,propylene, isobutylene, α-octene, α-dodecene, and α-octadecene;unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid,maleic acid, maleic anhydride, and itaconic acid and salts andmonoesters or diesters of these acids; nitrites such as acrylonitrileand methacrylonitrile; amides such as diacetoneacrylamide, acrylamide,and methacrylamide; olefinsulfonic acids such as ethylenesulfonic acid,allylsulfonic acid, and methallylsulfonic acid or salts of these acids;vinyl compounds such as alkyl vinyl ethers, dimethylallyl vinyl ketone,N-vinylpyrrolidone, and vinyl chloride; substituted vinyl acetates suchas isopropenyl acetate and 1-methoxyvinyl acetate; and vinylidenechloride, 1,4-diacetoxy-2-butene, vinylene carbonate, and acetoacetylgroup-containing monomers.

Examples thereof further include polyoxyalkylene group-containingmonomers, such as polyoxyethylene (meth)allyl ether, polyoxyethylene(meth)acrylamide, polyoxypropylene (meth)acrylamide, polyoxyethylene(meth)acrylate, polyoxypropylene (meth)acrylate,polyoxyethylene(1-(meth)acrylamido-1,1-dimethylpropyl) ester,polyoxyethylene vinyl ether, polyoxypropylene vinyl ether,polyoxyethylene allylamine, polyoxypropylene allylamine, polyoxyethylenevinylamine, and polyoxypropylene vinylamine; and cationicgroup-containing monomers, such as N-acrylamidomethyltrimethylammoniumchloride, N-acrylamidoethyltrimethylammonium chloride,N-acrylamidopropyltrimethylammonium chloride,2-acryloxyethyltrimethylammonium chloride,2-methacryloxyethyltrimethylammonium chloride,2-hydroxy-3-methacryloyloxypropyltrimethylammonium chloride,allyltrimethylammonium chloride, methallyltrimethylammonium chloride,3-butenetrimethylammonium chloride, dimethyldiallylammonium chloride,and diethyldiallylammonium chloride.

A PVA-based resin containing about 1.6 to 3.5% by mol 1,2-diol bondsincorporated in the main chain thereof by conducting polymerization at atemperature of 100° C. or higher can be used.

The PVA-based resin thus obtained has a degree of saponification of 65to 87% by mol. The degree of saponification thereof is preferably 68 to83% by mol, especially preferably 69 to 81% by mol. Too low degrees ofsaponification thereof are undesirable because there are cases where thesaponification of side-chain diacyloxy moieties, which are formed when a3,4-diacyloxy-1-butene is used as a comonomer, is insufficient or wherethe resultant has reduced water solubility. Furthermore, too high or toolow degrees of saponification thereof are undesirable because it maygive a vinyl chloride-based resin which includes coarse particles or hasa widened particle size distribution. The term degree of saponificationin the invention is defined as the proportion of the number of moles ofhydroxyl groups to the sum (mol) of the modifying group parts, such asthose derived from a 3,4-diacyloxy-1-butene, and the vinyl ester, e.g.,vinyl acetate.

The average degree of polymerization of the PVA-based resin according tothe invention (determined in accordance with JIS K 6726) is preferably400 to 850, more preferably 500 to 850, especially 600 to 830. Too lowaverage degrees of polymerization thereof are undesirable because thisPVA-based resin may be insufficient in the ability to function as aprotective colloid. Conversely, too high average degrees ofpolymerization thereof are undesirable because there are cases where itgives a vinyl chloride-based resin showing reduced plasticizerabsorption.

The content of 1,2-diol components in the PVA-based resin according tothe invention is preferably 1 to 20% by mol, more preferably 1 to 8% bymol, especially preferably 1 to 6% by mol, in particular 2 to 6% by mol.In case where the content thereof is too low, a wet foam forms in theinitial period of suspension polymerization of vinyl chloride or a dryfoam forms in the middle and later periods of the polymerization tocause scale deposition on the inner wall of the polymerization vessel.The scale inhibits the removal of the heat of polymerization reaction toreduce productivity. Furthermore, a scale may detach from the inner wallof the polymerization vessel and come into the vinyl chloride-basedresin product to cause fish-eyes during molding. Too low contentsthereof are hence undesirable. On the other hand, too high contentsthereof are undesirable because there are cases where polymerizationbecomes instable under some suspension polymerization conditions to givea vinyl chloride-based resin including coarse particles or reduced inquality, e.g., plasticizer absorption.

Examples of methods for introducing 1,2-diol components into a PVA-basedresin include a method by copolymerization as in the invention and amethod in which polymerization is conducted at a high temperature toincrease the proportion of head-head bonds and thereby introduce intothe main chain as described above. In the latter method, however, thereare limitations on the amount thereof which can be introduced.Introduction in an amount of 3% by mol or larger is actually impossibleby the latter method. However, since the PVA-based resin according tothe invention is one produced by the former method, the content of1,2-diol components can be regulated to any desired value within therange shown above.

The PVA-based resin to be used in the invention may be a mixture withanother PVA-based resin of a different kind. Examples of the differentPVA-based resin include one differing in the content of 1,2-diolstructural units represented by the general formula (1), one differingin the degree of saponification, one differing in the degree ofpolymerization, and one differing in other comonomer ingredient.

The PVA-based resin according to the invention preferably is a PVA-basedresin containing a carbonyl group in a molecule. Processes for producingthis PVA-based resin containing a carbonyl group in the molecule are notparticularly limited. Examples thereof include: a method in which aPVA-based resin obtained by the method described above is oxidized withan oxidizing agent such as hydrogen peroxide; a method in which thepolymerization described above is conducted in the presence of achain-transfer agent containing a carbonyl group, such as an aldehyde orketone, and the resultant is saponified; a method in which thepolymerization described above is conducted in the presence of1-methoxyvinyl acetate or the like and the resultant is saponified; anda method in which air is bubbled into the system during thepolymerization described above to obtain polyvinyl acetate and this issaponified. Especially advantageous industrially is the method in whichthe polymerization is conducted in the presence of a chain-transferagent containing a carbonyl group, such as an aldehyde or ketone, andthe polyvinyl acetate obtained is saponified to obtain a PVA-based resincontaining a carbonyl group.

Examples of the chain-transfer agent include aldehydes such asacetaldehyde, propionaldehyde, n-butyraldehyde, benzaldehyde, andcrotonaldehyde and ketones such as acetone, methyl ethyl ketone,hexanone, and cyclohexanone. Preferred of these, from the standpoint ofease of the control of chain transfer from vinyl acetate to the carbonylcompound, are acetaldehyde, benzaldehyde, propionaldehyde, andn-butyraldehyde. Such chain-transfer agents may be used alone or incombination of two or more thereof.

The amount of the chain-transfer agent to be added is regulatedaccording to the chain transfer constant of the chain-transfer agent tobe added, the target degree of polymerization of the PVA-based resin,etc. In general, however, the amount thereof is preferably 0.05 to 5% byweight, more preferably 0.1 to 3% by weight based on the fatty acidvinyl ester-based monomer, e.g., vinyl acetate. The chain-transfer agentmay be added all at once in the initial period of the polymerization ormay be added in the course of the polymerization. By adding thechain-transfer agent by a desired method, the molecular weightdistribution of the PVA-based resin can be controlled.

The amount of carbonyl groups contained in the PVA-based resin accordingto the invention is preferably 0.005 to 0.3% by mol, more preferably0.01 to 0.2% by mol, especially 0.03 to 0.15% by mol. Too low contentsof carbonyl groups are undesirable because this PVA-based resin theability to function as a protective colloid, as a dispersant, isreduced. Conversely, to introduce excess carbonyl groups is undesirablebecause this results in a PVA-based resin having an exceedingly lowdegree of polymerization.

The ability of the PVA-based resin to function as a protective colloid,when it is used as a stabilizer for suspension polymerization, can becontrolled by regulating the amount of conjugated double bonds to beformed by subjecting vinyl alcohol or vinyl acetate structural unitseach adjoining such a carbonyl group to dehydration/acetic acid removaltherefrom. Generally used as indexes to the content thereof areabsorbances respectively at 215 nm [assigned to the structure—CO—CH═CH—], 280 nm [assigned to the structure —CO—(CH═CH)₂—], and 320nm. [assigned to the structure —CO—(CH═CH)₃—] in an ultravioletabsorption spectrum of a 0.1% by weight aqueous solution of thePVA-based resin. The PVA-based resin according to the inventionpreferably is one in which the absorbance at 280 nm is 0.005 or higher,especially 0.01 or higher. Too low values of this absorbance areundesirable because it results in insufficient stability in suspensionpolymerization to yield coarse particles or give a widened particle sizedistribution.

In producing the PVA-based resin according to the invention, it ispreferred to conduct alkali saponification in the polyvinyl acetatesaponification step in the presence of a solvent having a permittivityof 32 or lower. A more preferred permittivity range is 6 to 29, and anespecially preferred permittivity range is 12 to 28. The permittivityexceeding 32 is undesirable because the resultant PVA-based resin has areduced block character regarding acetic acid groups remaining in thefatty acid ester groups in the resin. There are hence cases where thisPVA-based resin is insufficient in the ability to function as aprotective colloid when used as a stabilizer for suspensionpolymerization of vinyl chloride-based resin. The resultant vinylchloride-based resin may include coarse particles or have a widenedparticle size distribution.

Examples of the solvent having a permittivity of 32 or lower includemethanol (31.2), methyl acetate/methanol=1/3 (27.1), methylacetate/methanol=1/1 (21.0), methyl acetate/methanol=3/1 (13.9), methylacetate (7.03), isopropyl acetate (6.3), trichloroethylene (3.42),xylene (2.37), toluene (2.38), benzene (2.28), and acetone (21.4).Preferred of these are the methyl acetate/methanol mixed solvents.

A method of suspension polymerization of a vinyl-based compound with thedispersion stabilizer of the invention is explained below with respectto the polymerization of a vinyl chloride-based monomer as an example.The term vinyl chloride-based monomer means not only vinyl chloridealone but also a mixture of at least 50% by weight vinyl chloride andone or more other monomers copolymerizable therewith.

For practicing the method of suspension polymerization with thedispersion stabilizer of the invention, any technique used in theordinary suspension polymerization of vinyl chloride-based monomers canbe employed.

With respect to methods for adding the PVA-based resin as the dispersionstabilizer of the invention to the polymerization system in suspensionpolymerization, the PVA-based resin may be added as it is, i.e., as apowder, or in a solution form. The PVA-based resin may be added all atonce in the initial period of the polymerization or added divisionallyin the course of the polymerization.

The amount of the PVA-based dispersant to be used is not particularlylimited. In general, however, the amount thereof is preferably 5 partsby weight or smaller, more preferably 0.01 to 1 part by weight, evenmore preferably 0.02 to 0.2 parts by weight based on 100 parts by weightof the vinyl chloride-based monomer.

In suspension polymerization, the dispersion stabilizer of the inventionmay be used in combination with any of various known secondarydispersant.

Preferred secondary dispersant is lowly saponified PVA-based resinshaving a degree of saponification lower than 65% by mol and an averagedegree of polymerization of 100 to 750, in particular, ones having adegree of saponification of 30 to 60% by mol and an average degree ofpolymerization of 180 to 900.

Also usable is a secondary dispersant which is water-soluble orwater-dispersible and is a PVA-based resin having a low degree ofsaponification to which self-emulsifiability has been imparted byintroducing, e.g., ionic groups such as carboxylic acid groups orsulfonic acid groups into side chains or molecular ends of the resin.Examples thereof include secondary dispersant such as “GohsefimerLL-02”, “Gohsefimer L-5407”, “Gohsefimer L-7514”, “Gohsefimer LW100”,“Gohsefimer LW200”, “Gohsefimer LW300”, and “Gohsefimer LS210”(manufactured by The Nippon Synthetic Chemical Industry), and furtherinclude “LM-20”, “LM-25”, “LM-10HD” (manufactured by Kuraray Co., Ltd.),“Alcotex 55-002H”, “Alcotex WD100”, “Alcotex WD200”, “Alcotex 55-002P”(manufactured by Synthomer), “Sigma 404W”, “Sigma 202” (manufactured bySigma), and various secondary dispersant manufactured by CIRS.

The weight ratio of the addition amount of the PVA dispersion stabilizerof the invention to that of the secondary dispersant cannot becategorically specified because it varies depending on the kind of thedispersant, etc. However, the weight ratio thereof is preferably in therange of from 90/10 to 30/70, especially preferably 80/20 to 50/50.

By using a secondary dispersant in combination, not only the vinylchloride-based resin particles being obtained can be prevented fromcoming to have a thick skin layer on the surface thereof but also theaggregation of basic particles having a size on the order of one toseveral micrometers (primary particles) in each particle can becontrolled. As a result, a vinyl chloride-based resin further improvedin properties such as porosity distribution, plasticizer absorption, andmonomer removability is obtained.

As a suspension polymerization catalyst, any oil-soluble catalyst may beused. For example, a catalyst in use for the ordinary suspensionpolymerization of vinyl chloride-based monomers can be employed, such asbenzoyl peroxide, lauroyl peroxide, diisopropyl peroxydicarbonate,α·α′-azobisisobutyronitrile, α·α′-azobis-2,4-dimethylvaleronitrile,acetylcyclohexylsulfonyl peroxide, or a mixture thereof. Such catalystsmay be used alone or in combination of two or more thereof.

The dispersion stabilizer of the invention can be used in combinationwith another known dispersant, for example, a polymeric substance whichhas been used as a dispersion stabilizer for suspension polymerizationof a vinyl-based compound. Examples of such other dispersion stabilizersinclude PVA-based resins other than the PVA-based resin according to theinvention, such as PVA-based resins each having an average degree ofpolymerization of 100 to 4,500 and a degree of saponification of 65 to100% by mol, and derivatives of such PVA-based resins. Examples of thePVA-based resin derivatives include PVA formalized PVA, acetalized PVA,butyralized PVA, or urethanated PVA and PVA esters with a sulfonic acid,carboxylic acid, or the like. Examples of the other dispersionstabilizers further include the saponification product of copolymers ofa vinyl ester and a monomer copolymerizable therewith. Examples of thecopolymerizable monomer include olefins such as ethylene, propylene,isobutylene, α-octene, α-dodecene, and α-octadecene; unsaturated acidssuch as acrylic acid, methacrylic acid, crotonic acid, maleic acid,maleic anhydride, and itaconic acid or salts and mono- or dialkyl estersof these acids; nitrites such as acrylonitrile and methacrylonitrile;amides such as acrylamide and methacrylamide; olefinsulfonic acids suchas ethylenesulfonic acid, allylsulfonic acid, and methallylsulfonic acidor salts of these acids; and alkyl vinyl ethers, vinyl ketone,N-vinylpyrrolidone, vinyl chloride, and vinylidene chloride. However,the PVA-based dispersion stabilizer should not be construed as beinglimited to these examples.

Examples of the polymeric substance which is not the PVA-based resin andis known as a dispersant include cellulose derivatives such as methylcellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxypropylcellulose, hydroxybutyl methyl cellulose, hydroxyethyl cellulose,carboxymethyl cellulose, aminomethyl hydroxypropyl cellulose, andaminoethyl hydroxypropyl cellulose, starch, tragacanth, pectin, glue,alginic acid or salts thereof, gelatin, polyvinylpyrrolidone,polyacrylic acid or salts thereof, polymethacrylic acid or saltsthereof, polyacrylamide, polymethacrylamide, copolymers of vinyl acetateand an unsaturated acid such as maleic acid, maleic anhydride, acrylicacid, methacrylic acid, itaconic acid, fumaric acid, or crotonic acid,copolymers of styrene and any of these unsaturated acids, copolymers ofvinyl ether and any of those unsaturated acids, and salts or esters ofthese copolymers.

Various surfactants, inorganic dispersants, and the like may also besuitably used as an aid during the suspension polymerization.Furthermore, in the case of a degree of saponification lower than 65% bymol is used, it is possible to use the PVA-based resin according to theinvention as an aid.

In the suspension polymerization, a chain-transfer agent which has beenemployed in the ordinary suspension polymerization of vinylchloride-based monomers, such as mercaptoethanol or carbontetrachloride, can be used.

The temperature of the aqueous medium to be used in the suspensionpolymerization of a vinyl chloride-based monomer is not particularlylimited. Use may be made of hot water having a temperature of about 97°C., not to mention ordinary-temperature water of about 20° C. It is,however, preferred to use a method in which water which has been heatedbeforehand is used for the polymerization in place ofordinary-temperature water in order to shorten heating time forpolymerization (hot-charge method). When this method is employed, it ispreferred to use water which has been heated to 40 to 97° C. beforehand,preferably 40 to 85° C.

The polymerization temperature at the suspension polymerization may bearbitrarily selected from a range known to persons skilled in the artaccording to the target degree of polymerization of the vinyl-basedresin to be obtained. Usually, a temperature of 30 to 80° C. ispreferred. The suspension polymerization is conducted in a monomer/waterweight ratio generally in the range of 0.5 to 1.2. However, water may beadditionally added during the polymerization to compensate for aliquid-surface descent resulting from volume contraction by thepolymerization. This method is preferred because it is effective ininhibiting the formation of fish-eyes.

The polymerization pressure at the suspension polymerization may bearbitrarily selected from a range known to persons skilled in the artaccording to the target degree of polymerization of the vinyl-basedresin to be obtained and the polymerization temperature.

For stirring during the suspension polymerization, use can be made of astirrer which is not special but a known one which has been in generaluse in methods of the suspension polymerization of a vinylchloride-based monomer. Stirring blades in general use may be employed,such as Pfaudler blades, paddle blades, turbine blades, fan turbineblades, and brumargin blades. However, it is especially preferred to usePfaudler blades. Furthermore, there are no particular limitations oncombination with a baffle. Examples of the baffle include plate type,cylindrical, D type, loop type, and finger type baffles.

In the suspension polymerization of a vinyl chloride-based monomer, notonly the polymerization of vinyl chloride alone but also thecopolymerization of vinyl chloride with a monomer copolymerizabletherewith is conducted. Examples of the copolymerizable monomer includevinylidene halides, vinyl ether, vinyl acetate, vinyl benzoate, acrylicacid, methacrylic acid and esters thereof, maleic acid or its anhydride,ethylene, propylene, and styrene.

A polymerization regulator, anti-gelling agent, antistatic agent, pHcontroller, and the like which have been suitably used may be optionallyadded during the suspension polymerization of a vinyl chloride-basedmonomer.

By using the dispersant of the invention, foaming during suspensionpolymerization can be inhibited. In addition, a vinyl chloride-basedresin having excellent properties is obtained while preventing thequality characteristics (particle diameter, particle diameterdistribution, plasticizer absorption, etc.) of the vinyl chloride-basedresin from being influenced by the temperature of the added warm waterduring the polymerization.

Suspension polymerization with the dispersion stabilizer of thePVA-based resin of the invention was explained above mainly with respectto the polymerization of a vinyl chloride-based monomer. However, use ofthe PVA-based dispersion stabilizer of the invention should not beconstrued as being limited to the polymerization of vinyl chloride-basedmonomers. The stabilizer is usable also in the suspension polymerizationof any desired vinyl-based compounds such as aromatic vinyl compounds,e.g., styrene, acrylic acid or methacrylic acid and derivatives ofthese, vinyl ester compounds such as vinyl acetate, and ethylene/vinylacetate.

The dispersion stabilizer for suspension polymerization of the inventioncan be used also as a dispersion stabilizer in microsuspensionpolymerization. In this case also, the same satisfactoryfoaming-inhibiting effect is obtained. Monomers usable in thismicrosuspension polymerization are not particularly limited so long asthey are vinyl-based compounds. Examples thereof include vinyl halidecompounds such as vinyl chloride, aromatic vinyl compounds such asstyrene, vinyl ester compounds and copolymers thereof such as vinylacetate and ethylene/vinyl acetate, and acrylic acid or methacrylic acidand derivatives thereof.

EXAMPLES

The invention will be explained below by reference to Examples. However,the invention should not be construed as being limited to the followingExamples unless the invention departs from the spirit thereof.

In the following Examples and Comparative Examples, “parts” and “%” areby weight unless otherwise indicated.

Example 1

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,000 g of vinyl acetate, 100 g ofmethanol, 80 g (4 mol %) of 3,4-diacetoxy-1-butene, and 5.8 g ofacetaldehyde. Azobisisobutyronitrile was introduced thereinto in anamount of 0.05 mol % (based on the vinyl acetate added), and thetemperature thereof was elevated in a nitrogen stream with stirring toinitiate polymerization. At the time when the rate of polymerizationreached 90.5%, a given amount of m-dinitrobenzene was added to terminatethe polymerization. Subsequently, the vinyl acetate monomer remainingunreacted was removed from the system by bubbling methanol vapor intothe system. Thus, a methanol solution of a copolymer was obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) and methyl acetate (permittivity, 7.03) to regulate theconcentration thereof to 45% (permittivity of the saponificationsolvent, 24.0). This solution was added into a kneader. While thetemperature of the solution was kept at 40° C., a 2% methanol solutionof sodium hydroxide was added to the copolymer solution in an amount of5 mmol per mol of the sum of the vinyl acetate structural units and3,4-diacetoxy-1-butene structural units in the copolymer to conductsaponification. As the saponification proceeded, a saponificationproduct began to precipitate. At the time when the product became aparticle form, it was taken out by filtration, sufficiently washed withmethanol, and dried in a hot-air drying oven. Thus, a PVA-based resinwas obtained.

The PVA-based resin obtained was examined for property values by thefollowing methods. The results obtained are shown in Table 1.

[Degree of Saponification]

The degree of saponification of the PVA-based resin obtained wasdetermined from the amount of an alkali consumed by the hydrolysis ofresidual vinyl acetate structural units.

[Average Degree of Polymerization]

The average degree of polymerization of the PVA-based resin obtained wasdetermined in accordance with JIS K 6726.

[Content of 1,2-Diol Structural Units]

The content of 1,2-diol structural units in the PVA-based resin obtainedwas determined by completely saponifying the PVA-based resin, analyzingby ¹H-NMR, and calculating the content of the units. For the NMRmeasurement, use was made of “AVANCE DPX400”, manufactured by BrukerJapan.

[Content of Carbonyl Group]

It was determined in the following manner based on the method describedin Koubunshi Kagaku, Vol. 15, No. 156, pp. 249-254 (1958). The PVA-basedresin obtained was completely saponified and converted to a PVAhydrazone using p-nitrophenylhydrazine. Thereafter, an aqueous solutionthereof was examined for absorbance at 405 nm, and the content wascalculated therefrom.

[Absorbance]

The absorbance at 280 nm of a 0.1% by weight of the PVA-based resinobtained was determined with “Ultraviolet/Visible/Near-InfraredSpectrophotometer V-560”, manufactured by JASCO Corporation, using asample vessel (cell) having a thickness of 1 cm.

Subsequently, the PVA-based resin obtained was evaluated for thefollowing properties.

<Foaming of Aqueous Solution>

Into a measuring cylinder having a capacity of 1 L was introduced 200 mLof a 1% aqueous solution of the PVA-based resin. The temperature of thesolution was regulated to 40° C. Thereafter, a diffuser stone was placedin a liquid-bottom part, and air was bubbled into the solution at 0.2L/min for 5 minutes to foam the solution. After the air bubbling wasstopped, the time period required for the foam to disappear completelywas measured and evaluated based on the following. The results obtainedare shown in Table 1.

A: Disappeared in 8 minutes.

B: Disappeared in a period of more than 8 minutes and 10 minutes orless.

C: Disappeared in a period of more than 10 minutes and 30 minutes orless.

D: Not disappeared in 30 minutes.

<Suspension Polymerization of Vinyl Chloride>

Into a polymerization vessel equipped with a reflux condenser and havinga capacity of 2,000 L were added 450 g of the PVA-based resin obtained,260 g of di-2-ethylhexyl peroxydicarbonate, 900 kg of deionized water,and 600 kg of vinyl chloride monomer. Hot water was passed through ajacket to heat the contents to 57° C. with stirring to initiatepolymerization. The internal pressure of the polymerization vessel atthe time of polymerization initiation was 7.0 kg/cm²G. At the time whenthe pressure of the polymerization vessel decreased to 6.0 kg/cm²G, themonomer remaining unreacted was recovered. The resultant polymer slurrywas taken out of the vessel, dehydrated, and dried to obtain a vinylchloride-based resin.

The vinyl chloride-based resin obtained was evaluated for the followingproperties. The results obtained are shown in Table 2.

[Particle Size Distribution]

20 kg of the vinyl chloride-based resin obtained was classified with JISstandard sieves (JIS Z 8801) to determine the contents (wt %) of coarseparticles which did not pass through a 42-mesh sieve and of fineparticles which passed through a 250-mesh sieve.

[Scale Deposition State (Dry-Foam Formation State)]

The formation state of a dry foam in the polymerization vessel wasexamined by observing the reflux condenser for scale deposition thereon,and evaluated based on the following.

A: No scale deposition was observed.

B: Scale deposition was observed in a small amount.

C: Scale deposition was observed in a large amount.

[Amount of Foamy Polymer]

10 kg of the vinyl chloride-based resin was examined using a JISstandard sieve (JIS Z 8801) for the content (wt %) of coarse particleswhich did not pass through a 48-mesh sieve.

[Bulk Density]

The bulk density of the vinyl chloride-based resin obtained was measuredin accordance with JIS K 6721.

[Initial Coloration and Heat Resistance]

100 parts of the vinyl chloride-based resin obtained was subjected toroll kneading at 140° C. for 10 minutes together with 35 parts of DOP(dioctyl phthalate), 1 part of epoxidized soybean oil, and 2 parts of abarium-zinc-based stabilizer. Thereafter, the resultant was molded into0.65 mm-thick sheets with an extruder. Subsequently, eight of the sheetswere stacked up and hot-pressed at 180° C. for 5 minutes to produce apressed plate. The surfaces of this pressed plate were visually examinedfor coloration, and evaluated based on the following (initialcoloration).

Furthermore, the pressed plate was allowed to stand in a 190° C. Geeroven for 50 minutes and then evaluated for coloration in the same manner(heat resistance).

A: No coloration was observed at all.

B: Slight coloration was observed.

C: Coloration was observed.

Example 2

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,000 g of vinyl acetate, 100 g ofmethanol, 164 g (8.2 mol %) of 3,4-diacetoxy-1-butene, and 1.6 g ofacetaldehyde. Azobisisobutyronitrile was added thereinto in an amount of0.05 mol % (based on the vinyl acetate added), and the temperature waselevated in a nitrogen stream with stirring to initiate polymerization.At the time when the rate of polymerization reached 91.5%, a givenamount of m-dinitrobenzene was added to terminate the polymerization.Subsequently, the vinyl acetate monomer remaining unreacted was removedfrom the system by bubbling methanol vapor into the system. Thus, amethanol solution of a copolymer was obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) and methyl acetate (permittivity, 7.03) to regulate theconcentration thereof to 45% (permittivity of the saponificationsolvent, 24.0). This solution was introduced into a kneader. While thetemperature of the solution was kept at 40° C., a 2% methanol solutionof sodium hydroxide was added in an amount of 6 mmol per mol of the sumof the vinyl acetate structural units and 3,4-diacetoxy-1-butenestructural units in the copolymer to conduct saponification. As thesaponification proceeded, a saponification product began to precipitate.At the time when the product became a particle form, it was taken out byfiltration, sufficiently washed with methanol, and dried in a hot-airdrying oven. Thus, a PVA-based resin was obtained.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Example 3

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,000 g of vinyl acetate, 100 g ofmethanol, 18 g (0.9 mol %) of 3,4-diacetoxy-1-butene, and 8.6 g ofacetaldehyde. Azobisisobutyronitrile was added thereinto in an amount of0.05 mol % (based on the vinyl acetate added), and the temperature waselevated in a nitrogen stream with stirring to initiate polymerization.At the time when the rate of polymerization reached 95.0%, a givenamount of m-dinitrobenzene was added to terminate the polymerization.Subsequently, the vinyl acetate monomer remaining unreacted was removedfrom the system by bubbling methanol vapor into the system. Thus, amethanol solution of a copolymer was obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) and methyl acetate (permittivity, 7.03) to regulate theconcentration thereof to 45% (permittivity of the saponificationsolvent, 24.0). This solution was added into a kneader. While thetemperature of the solution was kept at 40° C., a 2% methanol solutionof sodium hydroxide was added in an amount of 5 mmol per mol of the sumof the vinyl acetate structural units and 3,4-diacetoxy-1-butenestructural units in the copolymer to conduct saponification. As thesaponification proceeded, a saponification product began to precipitate.At the time when the product became a particle form, it was taken out byfiltration, sufficiently washed with methanol, and dried in a hot-airdrying oven. Thus, a PVA-based resin was obtained.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Example 4

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added-1,000 g of vinyl acetate, 400 g ofmethanol, and 90 g (4.5 mol %) of 3,4-diacetoxy-1-butene.Azobisisobutyronitrile was added thereinto in an amount of 0.08 mol %(based on the vinyl acetate added), and the temperature was elevated ina nitrogen stream with stirring to initiate polymerization. At the timewhen the rate of polymerization reached 94.7%, a given amount ofm-dinitrobenzene was added to terminate the polymerization.Subsequently, the vinyl acetate monomer remaining unreacted was removedfrom the system by bubbling methanol vapor into the system. Thus, amethanol solution of a copolymer was obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) and methyl acetate (permittivity, 7.03) to regulate theconcentration thereof to 45% (permittivity of the saponificationsolvent, 24.0). This solution was added into a kneader. While thetemperature of the solution was kept at 40° C., a 2% methanol solutionof sodium hydroxide was added in an amount of 5 mmol per mol of the sumof the vinyl acetate structural units and 3,4-diacetoxy-1-butenestructural units in the copolymer to conduct saponification. As thesaponification proceeded, a saponification product began to precipitate.At the time when the product became a particle form, it was taken out byfiltration, sufficiently washed with methanol, and dried in a hot-airdrying oven. Thus, a PVA-based resin was obtained.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Example 5

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,000 g of vinyl acetate, 500 g ofmethanol, 70 g (3.5 mol %) of 3,4-diacetoxy-1-butene, and 12.1 g ofacetaldehyde. Azobisisobutyronitrile was added thereinto in an amount of0.1 mol % (based on the vinyl acetate added), and the temperature waselevated in a nitrogen stream with stirring to initiate polymerization.At the time when the rate of polymerization reached 96.2%, a givenamount of m-dinitrobenzene was added to terminate the polymerization.Subsequently, the vinyl acetate monomer remaining unreacted was removedfrom the system by bubbling methanol vapor into the system. Thus, amethanol solution of a copolymer was obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) and methyl acetate (permittivity, 7.03) to regulate theconcentration thereof to 50% (permittivity of the saponificationsolvent, 24.0). This solution was added into a kneader. While thetemperature of the solution was kept at 40° C., a 2% methanol solutionof sodium hydroxide was added to the copolymer solution in an amount of5 mmol per mol of the sum of the vinyl acetate structural units and3,4-diacetoxy-1-butene structural units in the copolymer to conductsaponification. As the saponification proceeded, a saponificationproduct began to precipitate. At the time when the product became aparticle form, it was taken out by filtration, sufficiently washed withmethanol, and dried in a hot-air drying oven. Thus, a PVA-based resinwas obtained.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Example 6

A PVA-based resin was obtained in the same manner as in Example 1,except that for the saponification of the copolymer, a 2% methanolsolution of sodium hydroxide was added in an amount of 7 mmol per mol ofthe sum of the vinyl acetate structural units and 3,4-diacetoxy-1-butenestructural units in the copolymer.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Example 7

A PVA-based resin was obtained in the same manner as in Example 1,except that for the saponification of the copolymer, a 2% methanolsolution of sodium hydroxide was added in an amount of 4 mmol per mol ofthe sum of the vinyl acetate structural units and 3,4-diacetoxy-1-butenestructural units in the copolymer.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Example 8

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,200 g of vinyl acetate, 60 g ofmethanol, and 117 g (6.5 mol %) of glycerin monoallyl ether.Azobisisobutyronitrile was added thereinto in an amount of 0.1 mol %(based on the vinyl acetate added), and the temperature was elevated ina nitrogen stream with stirring to initiate polymerization. At the timewhen the rate of polymerization reached 74.2%, a given amount ofm-dinitrobenzene was added to terminate the polymerization.Subsequently, the vinyl acetate monomer remaining unreacted was removedfrom the system by bubbling methanol vapor into the system. Thus, amethanol solution of a copolymer was obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) to regulate the concentration thereof to 50% (permittivity of thesaponification solvent, 31.2). This solution was added into a kneader.While the temperature of the solution was kept at 40° C., a 2% methanolsolution of sodium hydroxide was added in an amount of 5 mmol per mol ofthe sum of the vinyl acetate structural units and 3,4-diacetoxy-1-butenestructural units in the copolymer to conduct saponification. As thesaponification proceeded, a saponification product began to precipitate.At the time when the product became a particle form, it was taken out byfiltration, sufficiently washed with methanol, and dried in a hot-airdrying oven. Thus, a PVA-based resin was obtained.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Example 9

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,000 g of vinyl acetate, 450 g ofmethanol, 20 g (1.5 mol %) of vinylethylene carbonate, and 3.3 g ofacetaldehyde. Azobisisobutyronitrile was added thereinto in an amount of0.05 mol % (based on the vinyl acetate added), and the temperature waselevated in a nitrogen stream with stirring to initiate polymerization.At the time when the rate of polymerization reached 94.8%, a givenamount of m-dinitrobenzene was added to terminate the polymerization.Subsequently, the vinyl acetate monomer remaining unreacted was removedfrom the system by bubbling methanol vapor into the system. Thus, amethanol solution of a copolymer was obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) and methyl acetate (permittivity, 7.03) to regulate theconcentration thereof to 45% (permittivity of the saponificationsolvent, 24.0). This solution was added into a kneader. While thetemperature of the solution was kept at 40° C., a 2% methanol solutionof sodium hydroxide was added in an amount of 7 mmol per mol of the sumof the vinyl acetate structural units and 3,4-diacetoxy-1-butenestructural units in the copolymer to conduct saponification. As thesaponification proceeded, a saponification product began to precipitate.At the time when the product became a particle form, it was taken out byfiltration, sufficiently washed with methanol, and dried in a hot-airdrying oven. Thus, a PVA-based resin was obtained.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Example 10

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,200 g of vinyl acetate, 60 g ofmethanol, and 98 g (5.5 mol %) of 2,2-dimethyl-4-vinyl-1,3-dioxolane.Azobisisobutyronitrile was added thereinto in an amount of 0.1 mol %(based on the vinyl acetate added), and the temperature was elevated ina nitrogen stream with stirring to initiate polymerization. At the timewhen the rate of polymerization reached 70%, a given amount ofm-dinitrobenzene was added to terminate the polymerization.Subsequently, the vinyl acetate monomer remaining unreacted was removedfrom the system by bubbling methanol vapor into the system. Thus, amethanol solution of a copolymer was obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) to regulate the concentration thereof to 50% (permittivity of thesaponification solvent, 31.2). This solution was added into a kneader.While the temperature of the solution was kept at 40° C., a 2% methanolsolution of sodium hydroxide was added in an amount of 4 mmol per mol ofthe vinyl acetate structural units in the copolymer to conductsaponification. As the saponification proceeded, a saponificationproduct began to precipitate and finally became a particle form. Thissaponification product was dispersed in 3-N hydrochloric acid (mixedsolvent of water/methanol=1/1) to conduct solvolysis of ketal structureat 60° C. The particles were taken out by filtration, sufficientlywashed with methanol, and dried in a hot-air drying oven. Thus, aPVA-based resin was obtained.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Comparative Example 1

A PVA-based resin was obtained in the same manner as in Example 1,except that for the saponification of the copolymer, a 2% methanolsolution of sodium hydroxide was added in an amount of 8 mmol per mol ofthe sum of the vinyl acetate structural units and 3,4-diacetoxy-1-butenestructural units in the copolymer.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Comparative Example 2

A PVA-based resin was obtained in the same manner as in Example 1,except that for the saponification of the copolymer, a 2% methanolsolution of sodium hydroxide was added in an amount of 3.5 mmol per molof the sum of the vinyl acetate structural units and3,4-diacetoxy-1-butene structural units in the copolymer.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

Comparative Example 3

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,000 g of vinyl acetate, 160 g ofmethanol, and 5.8 g of acetaldehyde. Azobisisobutyronitrile was addedthereinto in an amount of 0.04 mol % (based on the vinyl acetate added),and the temperature of the contents was elevated in a nitrogen streamwith stirring to initiate polymerization. At the time when the rate ofpolymerization reached 96.3%, a given amount of m-dinitrobenzene wasadded to terminate the polymerization. Subsequently, the vinyl-acetatemonomer remaining unreacted was removed from the system by bubblingmethanol vapor into the system. Thus, a methanol solution of a copolymerwas obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) and methyl acetate (permittivity, 7.03) to regulate theconcentration thereof to 45% (permittivity of the saponificationsolvent, 24.0). This solution was added into a kneader. While thetemperature of the solution was kept at 40° C., a 2% methanol solutionof sodium hydroxide was added in an amount of 5 mmol per mol of thevinyl acetate structural units in the copolymer to conductsaponification. As the saponification proceeded, a saponificationproduct began to precipitate. At the time when the product became aparticle form, it was taken out by filtration, sufficiently washed withmethanol, and dried in a hot-air drying oven. Thus, a PVA-based resinwas obtained.

Property values of the PVA-based resin obtained are shown in Table 1.The resin was evaluated in the same manners as in Example 1, and theresults obtained are shown in Table 2.

TABLE 1 Content of Content 1,2-diol of Degree of structural CarbonylFoaming of Comonomer saponification Degree of unit group aqueouscomponent (mol %) polymerization (mol %) (mol %) Absorbance solutionExample 1 DAcB 72.5 800 3.9 0.080 0.086 A Example 2 DAcB 74.6 800 8.20.015 0.022 A Example 3 DAcB 69.8 820 0.9 0.116 0.166 B Example 4 DAcB72.8 880 4.4 0.009 0.008 B Example 5 DAcB 71.8 380 3.2 0.176 0.289 BExample 6 DAcB 86.2 800 3.9 0.080 0.098 A Example 7 DAcB 65.5 800 3.90.080 0.072 B Example 8 GMAE 72.2 510 6.4 0.008 0.007 A Example 9 VEC81.5 820 1.5 0.054 0.062 B Example 10 DMVD 68.2 450 5.4 0.007 0.006 BComparative DAcB 88.2 800 3.9 0.080 0.115 B Example 1 Comparative DAcB64.3 800 3.9 0.080 0.054 C Example 2 Comparative — 71.8 800 — 0.0730.079 D Example 3 Note) DAcB: 3,4-diacetoxy-1-butene GMAE: glycerinmonoallyl ether VEC: vinylethylene carbonate DMVD:2,2-dimethyl-4-vinyl-1,3-dioxolane

TABLE 2 Particle size distribution Amount of (%) Scale foamed Particles250-Mesh deposition polymer Bulk Initial Heat above 42-mesh particlesstate (%) density coloration resistance Example 1 0 0 A 0.004 0.55 A AExample 2 0 0 A 0.005 0.53 A A Example 3 0.004 0 B 0.010 0.53 A AExample 4 0.002 0 A 0.007 0.54 A A Example 5 0.240 0.03 A 0.250 0.50 A AExample 6 0.190 0.04 A 0.009 0.55 A A Example 7 0.009 0.02 A 0.010 0.51A A Example 8 0.002 0.01 A 0.008 0.51 A A Example 9 0.007 0.01 B 0.0100.53 A A Example 10 0.008 0.02 B 0.010 0.50 A A Comparative 0.400 0.30 C0.550 0.53 C C Example 1 Comparative 0.300 0.40 C 0.470 0.46 B C Example2 Comparative 1.900 0.80 C 2.440 0.48 C C Example 3

Example 11

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,200 g of vinyl acetate, 60 g ofmethanol, 48 g (2 mol %) of 3,4-diacetoxy-1-butene, and 11.1 g ofacetaldehyde. Azobisisobutyronitrile was added thereinto in an amount of0.01 mol % (based on the vinyl acetate added), and the temperature waselevated in a nitrogen stream with stirring to initiate polymerization.At the time when the rate of polymerization reached 80.0%, a givenamount of m-dinitrobenzene was added to terminate the polymerization.Subsequently, the vinyl acetate monomer remaining unreacted was removedfrom the system by bubbling methanol vapor into the system. Thus, amethanol solution of a copolymer was obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) and methyl acetate (permittivity, 7.03) to regulate theconcentration thereof to 45% (permittivity of the saponificationsolvent, 22.2). This solution was added into a kneader. While thetemperature of the solution was kept at 40° C., a 2% methanol solutionof sodium hydroxide was added in an amount of 6 mmol per mol of the sumof the vinyl acetate structural units and 3,4-diacetoxy-1-butenestructural units in the copolymer to conduct saponification. As thesaponification proceeded, a saponification product began to precipitate.At the time when the product became a particle form, it was taken out byfiltration, sufficiently washed with methanol, and dried in a hot-airdrying oven. Thus, a PVA-based resin was obtained.

Property values of the PVA-based resin obtained are shown in Table 3.

The PVA-based resin obtained was used to conduct the microsuspensionpolymerization of vinyl acetate in the following manner, and wasevaluated for foaming state in this polymerization.

Into a 500-mL separable flask made of glass were added 125.9 parts ofion-exchanged water in which 5.9 parts of the PVA-based resin obtainedhad been dissolved and 74.5 parts of vinyl acetate in which 0.4 parts oflauroyl peroxide had been dissolved. The solutions were homogeneouslymixed together by means of a homogenizer to bring the mixture into amicrosuspension state. Thereafter, a reflux condenser and a stirrer wereattached to the separable flask. This reaction mixture was heated to 70°C. with stirring to initiate polymerization. The reaction mixture whichhad been heated to that temperature was visually examined for foamingstate. As a result, a foam was observed only at the gas/liquidinterface.

Comparative Example 4

Into a reaction vessel equipped with a reflux condenser, droppingfunnel, and stirrer were added 1,200 g of vinyl acetate, 60 g ofmethanol, and 18.3 g of acetaldehyde. Azobisisobutyronitrile was addedthereinto in an amount of 0.005 mol % (based on the vinyl acetateadded), and the temperature was elevated in a nitrogen stream withstirring to initiate polymerization. At the time when the rate ofpolymerization reached 79.5%, a given amount of m-dinitrobenzene wasadded to terminate the polymerization. Subsequently, the vinyl acetatemonomer remaining unreacted was removed from the system by bubblingmethanol vapor into the system. Thus, a methanol solution of a copolymerwas obtained.

Subsequently, the solution was diluted with methanol (permittivity,31.2) and methyl acetate (permittivity, 7.03) to regulate theconcentration thereof to 45% (permittivity of the saponificationsolvent, 22.2). This solution was added into a kneader. While thetemperature of the solution was kept at 40° C., a 2% methanol solutionof sodium hydroxide was added in an amount of 6 mmol per mol of thevinyl acetate structural units in the copolymer to conductsaponification. As the saponification proceeded, a saponificationproduct began to precipitate. At the time when the product became aparticle form, it was taken out by filtration, sufficiently washed withmethanol, and dried in a hot-air drying oven. Thus, a PVA-based resinwas obtained.

Property values of the PVA-based resin obtained are shown in Table 3.

The PVA-based resin obtained was used to conduct the microsuspensionpolymerization of vinyl acetate in the same manner as in Example 11 andevaluated in the same manner. As a result, a foam had reached thehighest part in the separable flask.

TABLE 3 Content of Content of Degree of 1,2-diol Carbonyl Comonomersaponification Degree of structural group component (mol %)polymerization unit (mol %) (mol %) Absorbance Example 11 DAcB 83.5 7101.9 0.151 0.224 Comparative — 83.8 760 — 0.245 0.347 Example 4 Note)DAcB: 3,4-diacetoxy-1-butene

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

This application is based on a Japanese patent application filed on Feb.2, 2005 (Application No. 2005-026793) and a Japanese patent applicationfiled on Feb. 2, 2006 (Application No. 2006-025219), the contentsthereof being herein incorporated by reference.

INDUSTRIAL APPLICABILITY

The dispersion stabilizer for suspension polymerization of a vinyl-basedcompound of the invention attains excellent dispersion stabilizationduring the suspension polymerization of vinyl chloride and, hence, givesvinyl chloride-based polymer particles having a sharp particle sizedistribution and a high bulk density. An aqueous solution thereof isless apt to foam and, hence, the stabilizer is effective in diminishingthe formation of a wet foam during polymerization and in diminishingdry-foam formation. The stabilizer hence inhibits formation of a polymerscale, which is causative of fish-eyes, and a foamy polymer.Furthermore, vinyl-based polymer particles with less coloring andexcellent in heat resistance are obtained with the stabilizer.Consequently, the dispersion stabilizer of the invention is industriallyextremely useful.

1. A dispersion stabilizer for suspension polymerization of avinyl-based compound, which comprises a polyvinyl alcohol-based resincomprising a 1,2-diol component at a side chain and having a degree ofsaponification of 65 to 87% by mol.
 2. The dispersion stabilizer forsuspension polymerization of a vinyl-based compound according to claim1, wherein the polyvinyl alcohol-based resin has a 1,2-diol structuralunit represented by the general formula (1):

[wherein R¹, R², and R³ each independently represents a hydrogen atom oran organic group; X represents a single bond or a bonding chain; and R⁴,R⁵, and R⁶ each independently represents a hydrogen atom or an organicgroup].
 3. The dispersion stabilizer for suspension polymerization of avinyl-based compound according to claim 1, wherein the polyvinylalcohol-based resin is obtained by saponifying a copolymer of a vinylester-based monomer and a compound represented by the general formula(2):

[wherein R¹, R², and R³ each independently represents a hydrogen atom oran organic group; X represents a single bond or a bonding chain; R⁴, R⁵,and R⁶ each independently represents a hydrogen atom or an organicgroup; and R⁷ and R⁸ each independently represents a hydrogen atom orR⁹—CO— (wherein R⁹ represents an alkyl group)].
 4. The dispersionstabilizer for suspension polymerization of a vinyl-based compoundaccording to claim 3, wherein the compound represented by the generalformula (2) is 3,4-diacyloxy-1-butene.
 5. The dispersion stabilizer forsuspension polymerization of a vinyl-based compound according to claim1, wherein the polyvinyl alcohol-based resin is obtained by saponifyinga copolymer of a vinyl ester-based monomer and glycerin monoallyl ether.6. The dispersion stabilizer for suspension polymerization of avinyl-based compound according to claim 1, wherein the polyvinylalcohol-based resin is obtained by saponifying and decarboxylating acopolymer of a vinyl ester-based monomer and a compound represented bythe general formula (3):

[wherein R¹, R², and R³ each independently represents a hydrogen atom oran organic group; X represents a single bond or a bonding chain; and R⁴,R⁵, and R⁶ each independently represents a hydrogen atom or an organicgroup].
 7. The dispersion stabilizer for suspension polymerization of avinyl-based compound according to claim 6, wherein the compoundrepresented by the general formula (3) is vinylethylene carbonate. 8.The dispersion stabilizer for suspension polymerization of a vinyl-basedcompound according to claim 1, wherein the polyvinyl alcohol-based resinis obtained by saponifying and performing solvolysis of ketal structureof a copolymer of a vinyl ester-based monomer and a compound representedby the general formula (4):

[wherein R¹, R², and R³ each independently represents a hydrogen atom oran organic group; X represents a single bond or a bonding chain; R⁴, R⁵,and R⁶ each independently represents a hydrogen atom or an organicgroup; and R¹⁰ and R¹¹ each independently represents a hydrogen atom oran organic group].
 9. The dispersion stabilizer for suspensionpolymerization of a vinyl-based compound according to claim 8, whereinthe compound represented by the general formula (4) is2,2-dialkyl-4-vinyl-1,3-dioxolane.
 10. The dispersion stabilizer forsuspension polymerization of a vinyl-based compound according to claim1, wherein the polyvinyl alcohol-based resin is obtained throughsaponification in the presence of a solvent having a permittivity of 32or lower.
 11. The dispersion stabilizer for suspension polymerization ofa vinyl-based compound according to claim 1, wherein the polyvinylalcohol-based resin contains a carbonyl group in a molecule.
 12. Thedispersion stabilizer for suspension polymerization of a vinyl-basedcompound according to claim 1, wherein the polyvinyl alcohol-based resinhas a degree of saponification of 68 to 83% by mol.
 13. The dispersionstabilizer for suspension polymerization of a vinyl-based compoundaccording to claim 1, wherein the polyvinyl alcohol-based resin has anaverage degree of polymerization of 400 to
 850. 14. The dispersionstabilizer for suspension polymerization of a vinyl-based compoundaccording to claim 1, wherein a content of the 1,2-diol component in thepolyvinyl alcohol-based resin is 1 to 8% by mol.
 15. The dispersionstabilizer for suspension polymerization of a vinyl-based compoundaccording to claim 2, wherein the polyvinyl alcohol-based resin isobtained by saponifying a copolymer of a vinyl ester-based monomer and acompound represented by the general formula (2):

[wherein R¹, R², and R³ each independently represents a hydrogen or anorganic group; X represents a single bond or a bonding chain; R⁴, R⁵,and R⁶ each independently represents a hydrogen atom or an organicgroup; and R⁷ and R⁸ each independently represents a hydrogen atom orR⁹—CO— (wherein R⁹ represents an alkyl group)].
 16. The dispersionstabilizer for suspension polymerization of a vinyl-based compoundaccording to claim 2, wherein the polyvinyl alcohol-based resin isobtained by saponifying a copolymer of a vinyl ester-based monomer andglycerin monoallyl ether.
 17. The dispersion stabilizer for suspensionpolymerization of a vinyl-based compound according to claim 2, whereinthe polyvinyl alcohol-based resin is obtained by saponifying anddecarboxylating a copolymer of a vinyl ester-based monomer and acompound represented by the general formula (3):

[wherein R¹, R², and R³ each independently represents a hydrogen atom oran organic group; X represents a single bond or a bonding chain; and R⁴,R⁵, and R⁶ each independently represents a hydrogen atom or an organicgroup].
 18. The dispersion stabilizer for suspension polymerization of avinyl-based compound according to claim 2, wherein the polyvinylalcohol-based resin is obtained by saponifying and performing solvolysisof ketal structure of a copolymer of a vinyl ester-based monomer and acompound represented by the general formula (4):

[wherein R¹, R², and R³ each independently represents a hydrogen atom oran organic group; X represents a single bond or a bonding chain; R⁴, R⁵,and R⁶ each independently represents a hydrogen atom or an organicgroup; and R¹⁰ and R¹¹ each independently represents a hydrogen atom oran organic group].
 19. The dispersion stabilizer for suspensionpolymerization of a vinyl-based compound according to claim 2, whereinthe polyvinyl alcohol-based resin is obtained through saponification inthe presence of a solvent having a permittivity of 32 or lower.
 20. Thedispersion stabilizer for suspension polymerization of a vinyl-basedcompound according to claim 2, wherein the polyvinyl alcohol-based resincontains a carbonyl group in a molecule.
 21. The dispersion stabilizerfor suspension polymerization of a vinyl-based compound according toclaim 2, wherein the polyvinyl alcohol-based resin has a degree ofsaponification of 68 to 83% by mol.
 22. The dispersion stabilizer forsuspension polymerization of a vinyl-based compound according to claim2, wherein the polyvinyl alcohol-based resin has an average degree ofpolymerization of 400 to
 850. 23. The dispersion stabilizer forsuspension polymerization of a vinyl-based compound according to claim,wherein a content of the 1,2-diol component in the polyvinylalcohol-based resin is 1 to 8% by mol.